The present invention is generally directed to an apparatus and processes thereof for the preparation of particulate materials with narrow particle size distribution properties. More specifically, the present invention relates to improved classifier chamber geometries, such as a reduced or variable fine particle outlet diameter, and which diameters enable a high level of control over the physics of the separation process in the classifier, such as the particle size and quality of the particle separation, and thereby provides unexpected and superior particle size separation quality in a classifier. Furthermore the present invention provides the componentry and a method for making a convenient retrofit or adaptation of an existing conventional classifier apparatus to provide the advantages of the present invention.
In particle processing arts, for example, for the preparation of fine and uniformly disperse particulate materials, there exists various equipment and mechanical processes for achieving selective separation of particulate powders into eligible and non-eligible particle size fractions or ranges, and are collectively referred to as classifiers and classification equipment and processes.
In the manufacture of particulate powders, such as electrostatographic toner compositions, a classifier apparatus employing a rotating wheel is commonly used to accomplish classification. In general, the rapidly rotating classifier wheel creates a dynamical fluid vortex which provides the necessary forces to achieve separation of particles greater than a certain size from particles less than a certain size.
The extent or sharpness of the separation of particles of different sizes achieved by the classifier is an important measure of the quality of the separation equipment and process, and is generally reflected in the quality of the resultant particles, for example, the physical performance characteristics and properties of the particles. The sharpness of the separation is also a measure of how well the classifier can discriminate among similarly sized particles. Ideally, a classifier will separate a feed particle stream containing a mixture of fine and coarse particles sizes into two distinct streams: a coarse stream and a fines stream with little or no overlap in size distribution.
The degree of sharpness of the separation is measured, for example, using a coarse grade efficiency calculation. The calculation indicates what fraction of particles with a certain size will travel to the coarse stream, and what fraction will travel to the fines stream. A ratio of the size at which 25 percent of the particles travel to the coarse stream (D25) and the size at which 75 percent of the particles travel to the coarse stream (D75) is used as a nominal measure of sharpness (D25/D75). An ideal separation provides a sharpness (D25/D75) equal to 1. In currently available commercial classification equipment, a sharpness index exceeding a value of 0.7, for example, from about 0.7 to about 1.0, is considered to be excellent and considered difficult to attain without exceptional effort and operating conditions. Other classification metrics include (D84/D50) which is referred to as Upper Geometric Size Distribution (UGSD). This metric is determined from the fines stream of a certain volume median and distribution resulting from the separation and is a good indicator of separation sharpness. In general, the lower the Upper Geometric Size Distribution for a certain volume median size, the higher the sharpness index.
Commercially available classifier wheels generally provide little or no profiling, or only provide a profile which maintains a constant wheel height or constant air flow radial velocity. These conditions typically result in a particle cut point situation which diminishes towards the particle outlet, and is believed to lead to an undesirable buildup of solids concentration in the free vortex region. This buildup of solids concentration in the free vortex region is believed to have a detrimental effect on the sharpness index.
The present invention in embodiments overcomes these and other problems encountered in the prior art.
Commonly owned and assigned U.S. Pat. No. 5,927,510, to Leute, et al., issued Jul. 27, 1999, discloses an apparatus for the classification of solid particulates entrained in a fluid, comprising: a housing provided with a feed inlet, a fine fraction outlet, and a coarse fraction outlet; and a classifier wheel having an upper and lower surface, and a plurality of blade vanes connecting the upper surface to the lower surface at the peripheral edges of the upper and lower surfaces, and wherein the wheel has a constant cut point geometry.
U.S. Pat. No. 5,244,481, issued Sep. 14, 1993, to Nied, discloses a vertical air separator with a rotating separator wheel upon which separating air loaded with fine goods flowing from outside towards the inside impinges, from which the separating air axially flows off through an outlet connection pipe in order to be guided to its further use, e.g. in a filter or the like, the separating wheel being provided with a down stream cover plate and a second cover plate being axially distance therefrom, and blades being disposed between the two cover plates at their periphery, and the outlet connection delivery end averted from the separating wheel emptying into an outlet chamber the cross section of which is distinctly larger than the cross section of the said outlet connection pipe so that there occurs an abrupt change of the cross section between the outlet connection pipe and the outlet chamber. A constant radial velocity wheel is described, wherein the airflow velocity is constant regardless of the radial position in the wheel, reference col. 7, lines 21-32.
U.S. Pat. No. 5,377,843, to Schumacher, issued Jan. 3, 1995, discloses a classifying wheel for a centrifugal-wheel air classifier, through which the classifying air flows from outside to the inside against its centrifugal action. The wheel has blades arranged in a ring extending parallel to the axis of rotation of the wheel. The blades are positioned between a circular disc carrying the classifying wheel hub and an annular cover disc. The classifying wheel is entirely made in one piece and of a wear-resistant sintered material. The flow channels of the classifying wheel are formed by the surfaces of the classifying wheel blades extending parallel to each other and in direction of the axis of rotation of the wheel. The cut point of the fine product can be precisely controlled by varying the rotational speed of the turbine. This maintenance free design produces unmatched sharpness in cut size. The lack of internal seals makes oversize xe2x80x9cleakagexe2x80x9d impossible and allows air flows to be maximized resulting in extremely high product yields.
U.S. Pat. No. 5,366,095, to Martin, issued Nov. 22, 1994, discloses an air classification system comprised of dual cylindrical chambers mechanically separated, to allow a zone of atmospheric air in between. A primary classification chamber is situated vertically below a concentric secondary classification chamber. A rotating parallel blade turbine is situated within the lower primary chamber in order to effect centrifugal particle classification upon a feed material intimately mixed in an air stream. A tubular rotary discharge connected to the turbine which passes through the zone of atmospheric air separating the dual chambers, and extends into the upper secondary chamber which exits to collect and discharge the classified product from the system. A classifier of this design is capable of separating ultra fine particles without stray amounts of oversize with extremely high fine product yields.
The aforementioned references are incorporated in their entirety by reference herein.
In the particle separation and classification processes of the prior art, various significant problems exist, for example, difficulties in predicting or controlling both the particle size and particle size distribution of the particulate products produced. Other disadvantages associated with the prior art methods for separating particulate materials are that they typically provide products with highly variable particle size and or particle size distribution properties. These and other disadvantages are avoided, or minimized with the apparatus and processes of the present invention.
Thus, there remains a need for particle separation apparatus and processes, which provide for the simple and inexpensive preparation, separation, and classification of the particulate material, for example, pigmented resin particles used in dry toner and liquid ink applications. Practitioners in the art have long sought an inexpensive, efficient and environmentally efficacious means for producing narrow particle size distributions using conventional classification and separation equipment, having operator controllable or selectable particle size and particle size distribution properties.
Embodiments of the present invention, include:
An article comprising:
a disk with a circular opening in the center of the disk; and
a fastener adapted to concentrically attach the disk to the particle outlet opening of a classifier wheel;
A modified classifier wheel comprising:
an upper solid surface and a lower surface with a first circular opening therein;
a plurality of blade vanes connecting the upper surface to the lower surface at the peripheral edges of the upper and lower surfaces, and
an article as described above which is fixed to the lower surface which forms a second circular opening within the first circular opening and effectively reduces the diameter of the first circular opening;
An apparatus for the classification of solid particulates entrained in a fluid, comprising:
a housing provided with a feed inlet, a fine fraction outlet, and a coarse fraction outlet; and
a modified classifier wheel as described above;
A process for separating and classifying particulates including:
providing an apparatus as described above;
rotating the classifier wheel at speed of from about 500 to about 25,000 revolutions per minute, and preferably from about 500 to about 5,000 revolutions per minute; and
introducing to the apparatus a solid particle feed comprising a fluid stream containing particulates of from about 0.1 to about 1,000 microns in diameter, wherein the fine particles in the particle feed move toward the center of the wheel and thereafter exit the classifier wheel and housing via the fine fraction outlet opening, and the coarse particles move toward the periphery of the wheel and exit the wheel via the coarse fraction outlet; and
A kit for retrofitting an existing classifier wheel to enable embodiments of the present invention wherein the kit comprises:
a disk with a circular opening in the center of the disk; and
at least one fastener adapted to attach the disk to the fine particle outlet of an a existing classifier wheel.
These and other embodiments of the present invention are illustrated herein.